The present invention relates to an optical disc apparatus such as an opto-magnetic disc apparatus for recording and reproducing information by a light beam.
In general, laser recording and reproduction apparatus (hereinafter termed optical pickups) used in the opto-magnetic disc apparatus have a complex optical system for signal detection and thus become too heavy in weight for high speed data access.
There is known a separate type of optical system in which an objective lens, an actuator to drive it, and a prism for guiding the light beam to the objective lens and the like configure a feed mechanism, that is a movable unit which moves in the direction of the radius of a disc while other portions of the optical pickup comprise a fixed unit which is fixed to a base of a disc apparatus so that it is possible for the weight of the movable unit to be greatly reduced and so that the access time and the power consumption when there is access are both reduced.
FIG. 1A and 1B are configuration diagrams showing one example of a conventional opto-magnetic disc apparatus, with FIG. 1A being a side view, and FIG. 1B being a plan view.
As is shown in the figures, an opto-magnetic disc apparatus 31 of the conventional type is configured from an opto-magnetic disc 2 housed in a cartridge 3, a feed apparatus 5, a base 34, an optical pickup 36 and the like.
As is shown in FIG. 1A, a laser beam 12 that is irradiated from a fixed unit 36-1 of the optical pickup 36 fixed to the base 34, is irradiated to a moving unit 6-2 of the optical pickup 36 and which can move in the direction shown by an arrow 5A due to the feed apparatus 5, has its direction changed to the direction perpendicular to it's original direction by a guiding prism 13, is focussed by an objective lens 14, and is irradiated to the opto-magnetic disc 2 in the `just-focussed` status by an objective lens drive actuator 15.
In the separate type of optical system in such opto-magnetic disc apparatus 31, the configuration is such that the direction of motion shown by the arrow 5A (the feed direction) of the moving unit 6-2 and the optical axis of the laser beam 12 irradiated from the fixed unit 36-1 must always be kept in parallel with each other and this imposes a significant restriction on the optical path design of the optical pickup 36.
As shown in FIG. 1B, when there is reproduction, the light beam irradiated from a semiconductor laser 7 undergoes high-frequency modulation at the high-frequency superimposing circuit 8 that is disposed to the rear of the semiconductor laser 7, and after it has been collimated by a collimator lens 9, is irradiated to a beam shaping prism 40. The light beam irradiated from the semiconductor laser 7 has a cross section that is approximately elliptical in shape and this cross section is shaped into an approximately circular shape by the beam shaping prism 40. The light beam so shaped is irradiated via a beam splitter 11, to the guiding prism 13, where it is reflected and guided to the objective lens 14. The objective lens 14 then focuses the light beam to the opto-magnetic disc 2.
The light that is reflected from the opto-magnetic disc 2 is irradiated via the objective lens 14 and the guiding prism 13 to the beam splitter 11 where it is reflected by a splitting surface 11A and irradiated in the direction of a signal detection system. More specifically, this reflected light beam passes through a 1/2 wavelength plate 16 and then one portion of it is reflected by a polarizing beam splitter 17, passes through a total reflection prism 18, a condenser lens 19A and a cylindrical lens 20, is converted into an electrical signal by a four-divided photodiode 21A and is outputted a detection signal. On the other hand, the remaining portion of the light beam passes through the polarizing beam splitter 17, condenser lens 19B, is converted into an electrical signal by a four-divided photodiode 21B and is outputted as another detection signal. And a reproduced RF signal is obtained as the difference between them. These photo-diodes 21A and 21B are configured so that they are both arranged on the same side inside the fixed unit 36-1.
In a separate type of optical system described above, (i) the moving unit 6-2 must not interfere with the fixed unit 36-1 even if it moves to the outermost portion of the opto-magnetic disc 2, (ii) the optical system must be adjustable even though the disc 2 is mounted and and (iii), the moving direction of the moving unit 6-2 shown by the arrow 5A must be parallel with the optical axis of the laser beam 12 that is irradiated from the fixed unit 36-1. Therefore as is clear from FIG. 1B, the fixed unit 36-1 must protrude from at least one direction from the disc 2 or the cartridge 3. For example, in the example shown in the figure, the fixed unit 36-1 protrudes with respect to the cartridge 3 by the dimension L.sub.3 in the direction of motion shown by the arrow 5A.
Accordingly, as is clear from the figure, an optomagnetic disc apparatus 31 using such a separated type of optical system cannot have the length in the direction of motion shown by the arrow 5A made shorter than the diameter of the opto-magnetic disc 2 (or the length of the cartridge 3) to which the length L3 of the amount of protrusion from the fixed unit 36-1 has been added.
Furthermore, the width of the opto-magnetic disc apparatus 31 in the direction of width shown by an arrow W perpendicular to the direction of motion shown by the arrow 5A must have the length longer than the diameter of the opto-magnetic disc 2 (or the width W.sub.2 of the cartridge 3) and the width W.sub.3 of the fixed unit 36-1.
In consideration of this design condition, this opto-magnetic disc apparatus 31 is configured so that the width W.sub.3 of the fixed unit 36-1 is less than the width W.sub.2 of the cartridge 3 and as shown in FIG. 1B, so that the right surface of the fixed unit 36-1 is within the surface of the right side of the cartridge 3.
On the other hand, in order to improve the accuracy of the focus servo operation by the astigmatism method, as is known, it is advantageous to have a large focal length for the condenser lens 19A described above, in order to focus to the 1/4 photodiode 21A.
However, this design condition for a conventional optical disc apparatus 31 having the configuration described above cannot have a large focal length for the condenser lens 19A and so there is the problem that the focus servo operation has a poor accuracy.